add Confit and start config changeover

This adds a snapshot of the current Confit source (not a crime because Confit is
currently unreleased). It also changes around the bootstrapping mechanisms
enough to let "beet ls" run with the new Confit-based configuration. There's
much more to do.
confit: fix unicode and numeric validation
eradicate config_val from most built-in commands
replacements and path formats as ordered pairs
sync with confit validation redesign
begin moving importer/autotagger to confit
merge fixes from master
the ImportSession class

ImportSession will replace ImportConfig. It will have a *sane* number of fields
that are specific to the particular invocation of the importer -- e.g., lib and
paths. It also has a little bit of logic attached. Finally, it provides a method
for hooking the callbacks into the UI that is more elegant than assigning
callback functions -- OO inheritance to the rescue!
merge fixes from master
merge fixes and additions from master
remove some lingering ImportConfig references
remove config argument in command function sigs
fix calls to _pick_format
merge latest confit
fix a few old config references in importer
track down a few more misuses of confit
merge fix from master
fix outdated config references
clean up outdated FIXMEs
address a few leftover FIXMEs for confit